Photovoltaic (PV) cells are known semiconductor devices that convert light (i.e. by photons impinging on a pn junction) into electrical energy. Of course, sunlight can be used as the source of energy. Amorphous silicon, crystalline silicon, and selenium are examples of materials that are used in devising such cells. The voltage provided by an individual cell may be relatively small but many such cells can be combined as modules in electrical series and/or parallel connections to produce electrical power at voltage and current levels suitable for many applications. The modules are often constructed with flat surfaces to receive the incident light.
Solar hydrogen generation by photovoltaic-electrolyzer (PV-electrolyzer) systems is a potentially important, renewable and environmentally beneficial energy source for hydrogen fueled devices such as fuel cells. Planar modules of clusters of photovoltaic cells can be arranged to produce direct current voltage and current levels for a system of electrolysis cells to produce hydrogen and oxygen from water. In other words, an electrolysis system can be devised to deliver hydrogen gas at a required or design rate. And a photovoltaic system can be designed to provide electrical power for the specified electrolysis system. However, there is a challenge in the design and operation of a photovoltaic system because of the large variation in solar radiant flux density (irradiance) at virtually every location on the surface of the earth.
The planar module, or cluster of modules which is called an array, represents a relatively high investment cost per unit of required power and they require substantial land area in which to receive sunlight. If the PV-electrolysis systems are to be located in populated areas their size is a critical design consideration. So they must be operated to make good use of available sunlight.
Photovoltaic modules are typically installed as arrays of modules with a fixed orientation depending on the site characteristics and cost constraints. One orientation that is used on flat roofs is the so-called horizontal configuration in which the modules face straight up towards the sky. Another fixed configuration, that is considered the best overall fixed configuration for PV installations in North America, is one in which the modules face south and are tilted with respect to the ground at an angle equal to the site latitude. For example, for Detroit, with a latitude of approximately 42 degrees north of the equator, the modules would be tilted at a 42 degree angle with respect to the ground. The angle between the sun's position and the surface of the earth is called the solar altitude angle. Some references recommend using a module tilt angle equal to 90% of the latitude, e.g. a tilt of 38 degrees for Detroit, since this gives higher PV energy output in the summer, when there is more solar energy available. However, this configuration would give less solar energy in the winter, so it may or may not be superior depending on the seasonal energy needs of the user.
On sunny days, so-called two-axis solar tracking—continually orienting the solar modules perpendicular to the rays of the sun throughout each day of the year—produces the maximum energy. This is because the response of a solar module to a ray of light is proportional to the cosine of the angle between a line perpendicular to the module surface and the solar ray impinging on the surface. If the solar radiation is perpendicular to the surface, the maximum power for a given solar flux will be obtained (cosine 0°=1). For solar radiation impinging at 90 degrees from the normal, no power will be produced (cosine 90°=0). While two-axis solar tracking keeps the planar module facing the sun, it does not take into account the variation in solar irradiance due to atmospheric cloud cover and variation in the cloud coverage.
This invention provides a method of operation for a PV module under varying atmospheric conditions, continually positioning the module to make good use of sunlight in both cloudless and cloudy conditions.